29.1 Introduction

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Seismic isolation and vibration-control systems are relatively new and sophisticated concepts that require

more extensive design and detailed analysis than do most conventional seismic designs of structures.

In general, these systems will be most applicable to structures whose designers seek superior

earthquake performance. Seismic base isolation and passive energy-dissipation systems are viable design

strategies that have already been used for seismic protection of a number of structures. Other special

seismic protective system techniques such as active control, semiactive control, hybrid combinations of

active and passive devices, and tuned mass and liquid dampers may also provide practical solutions in the

29-1

© 2005 by Taylor & Francis Group, LLC

near future. An innovative challenge is highly expected in this field for the seismic safety enhancement of

civil structures.

29.1.1 From Ductility Design to Base Isolation and Control Design

The conventional method of seismic design mainly

deals with increasing capacity. The approach is

based on designing a strong and ductile structure

(see Figure 29.1), which can take care of the inertial

forces generated by the earthquake shaking. The

approach results in increasing the size of structural

members and connections, and providing

additional bracing members and shear walls, or

other stiffening members. The increase in stiffness

then attracts more seismic forces and in turn

requires further strengthening, which becomes

uneconomical. Therefore, the conventional practice

permits safe design of a structure on the

premise that inelastic action in a ductility-based

designed structure will dissipate significant energy

and enable it to survive a severe earthquake without collapse. The conventional designs may permit

some structural damage because of inelastic deformation in the members and also in nonstructural

elements. Contents of structure can get damaged due to large interstory drift and high-floor

accelerations.

It is difficult to control structural damage and it may be dangerous in unexpected strong seismic

events. It has been observed that, in the event of major seismic events, structures based on the

conventional design methods suffered damage, experienced high-floor accelerations, and resulted in

disruption of essential services such as transportation, communication, and so on. Thus, for the class of

structures like nuclear power plants, museums, hospital buildings, buildings with artifacts, important

bridges, and such structures located in high-seismicity regions, this ductility-based design is not suitable.

The need to minimize earthquake damage in critical and important structures prompted civil engineers

to search for other methods of earthquake-resistant designs, which can not only protect structures from

earthquake motions but also keep them functional during and after strong earthquakes. To this end, base

isolation and structural control methods are found to be a solution.

Base isolation has the capability to reduce the seismic response of a structure by isolating it from the

ground shaking (Figure 29.2a). An isolation system reduces the transmission of ground vibration,

thus enabling the structure to experience less shaking from the ground. Therefore, structural damage and

occupants’ inconvenience can be minimized using this technique. However, at the expense of safety

and the convenience of structure, the bearings undergo significant drift during large earthquakes

that may disrupt the function of the bearings themselves and supply lines of services such as water

and gas.

Another way of reducing seismic response is by using the structural control method. It has the

capability of modifying the structural properties, such as stiffness, mass, and damping, and providing

passive or active counterforces. Figure 29.2b shows the schematic diagram of the structural

control method in a civil structure. It shows some examples of devices generally used for applying

control forces.

The seismic safety enhancement of structures using the structural control method can be

categorized as active and passive systems. There are also hybrid systems that represent combinations

of active and passive, and semiactive systems to represent active controller that employs controllable

passive devices.

Plastic hinges in a

ductile structure,

enabling the whole

structural system to

absorb seismic energy

Ground excitation

FIGURE 29.1 Schematic of a structure with ductile

members.

29-2 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

Owing to changes in code provisions or upgradation of seismic zones, many structures come into the

category of “unsafe” and require retrofitting. Response control strategies are found to be easier than other

options, economical, and are often the only alternative for such cases.

29.1.2 The Importance of Reducing Seismic Input and Response

As mentioned above, by using the conventional method of seismic design, the design may permit some

structural damage because of inelastic deformation in the members, and also in nonstructural elements,

during large earthquakes. The ductility enables the structure as a whole to absorb the seismic energy.

Once the structural response goes deeply into the plastic range during a large earthquake, structures may

not be operational or repairable.

If the seismic input to the structure and structural response can be reduced, then the structural damage

can be minimized. For higher reliability of structures even under very severe earthquake motion,

structural control techniques that effectively reduce seismic force to structures are developed. The fast

development of technology, particularly in the fields of electronics and computer science, has provoked

the researchers in some centers worldwide to intensify development of a new concept with the new

philosophy of seismic design. Generally, this concept is known as a design of intelligent structures or

smart structures.

Owing to the experience of severe damage due to the Kobe earthquake, public demand for seismic

performances of civil infrastructures became relatively clear in Japan. Civil infrastructures are

constructed with the tax paid by the public, so a collapse or near collapse with unrepairable damage

cannot be accepted, even under a very rare earthquake loading. Infrastructures are also expected to serve

as public tools to help rehabilitate the affected society. For this purpose, infrastructures have to be

repaired in a relatively short time, even though their functions are temporarily terminated due to severe

earthquake loading.

Base Isolation

Bearings

Ground shaking

Structure response

Active Varying

Stiffness

Joint

Damper

Hysteretic Type

Damping

Variable

Damping

Moving Mass

Control

(a) (b)

FIGURE 29.2 Schematic of a civil structure with (a) isolation bearings and (b) the structural control method.

Seismic Base Isolation and Vibration Control 29-3

© 2005 by Taylor & Francis Group, LLC

The public demand for seismic performance objectives of infrastructures shows that structural damage

has to be limited even against very rare earthquake loading (Figure 29.3). The figure shows that civil

infrastructures must be fully operational during and after frequent, weak earthquakes. They also expected

to be operational even after very rare, strong earthquakes. To achieve the objectives, new technologies are

to be developed that can result in the desired performance.

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